1-Chloro-1, 1-difluoroethane is used particularly as an intermediate in the synthesis of 1,1-difluoroethylene, CF.sub.2 .dbd.CH.sub.2 (itself a monomer of increasing importance for the industrial production of fluorinated polymers), but also as an aerosol propellant. It is generally prepared by reacting 1,1,1-trichloroethane with hydrofluoric acid in the liquid phase.
In a known process (German Patent No. 2,137,806), the reaction can be carried out without a catalyst provided the temperature is high (110.degree. C.) and there is a large excess of hydrofluoric acid (molar ratio HF/C.sub.2 H.sub.3 Cl.sub.3 between 15 and 30). This gives a substantial amount of 1,1,1-trifluoroethane as a by-product and results in a low productivity in terms of 1-chloro-1,1-difluoroethane. Furthermore, the entrainment of hydrofluoric acid by the hydrochloric acid formed in this chlorine .rarw..fwdarw. fluorine exchange reaction makes it necessary to use specially adapted equipment (under a sufficient pressure) to recover this expensive raw material. The patent mentioned above indicates that the use of HSO.sub.3 F results in the formation of tars, probably due to the chemical instability of this catalyst (oxidation by SO.sub.3).
Other known processes recommend the use of catalysts based on antimony or molybdenum derivatives. See, for example, German Patent No. 2,659,046 and Japanese publication Nos. 74-03965, 76-29404 and 76-39606.
The major disadvantage of antimony derivatives is the fact that they are rapidly deactivated by reduction to the lower oxidation state Sb.sup.3+. The answer to this, which consists of continuous oxidation of the catalyst (with chlorine), contributes to the formation of heavy compounds (derived from chlorination of the CH.sub.3 group of the 1,1,1-trichloroethane). Furthermore, the solubility of some catalytic species (for example SbF.sub.3) in the reaction medium is so low that it usually causes physical separation of this species and renders the action of the chlorine ineffective. As a result, the chemical reaction is non-uniform, a large amount of catalyst is consumed. There is sustained corrosion due to the formation of superacidic compounds which are very aggressive towards the reactor material. Finally, the difficulty of suitably controlling the catalytic activity leads to the undesirable formation of a substantial amount of 1,1,1-trifluoroethane as a by-product.
With molybdenum derivatives, the formation of volatile species results in a loss of catalyst in the gas phase of the reactor.
The preceding references are incorporated by reference.